The present invention relates to hydraulic apparatus such as hydrostatic steering controllers which have relatively movable valve members for directing high pressure fluid from a source to a use location such as a hydraulic steering motor. The present invention relates particularly to a valve structure in which a pressure dam is formed for limiting leakage of high pressure fluid being directed to a use location by the valve structure.
It is known to provide a hydrostatic steering controller with pressure dam grooves for blocking leakage of high pressure fluid being directed by the controller to a hydraulic steering motor. One known type of prior art controller which forms pressure dams is U.S. Pat. No. 3,600,893. The controller disclosed in this patent includes an axially movable control valve member movable relative to a sleeve valve member. The sleeve valve member has a pair of cylinder grooves connected with the chambers of a hydraulic steering motor and a return groove between the cylinder grooves. A pressure dam groove is disposed in the sleeve valve member between each cylinder groove and the return groove. Pressure fluid is directed from the source to both pressure dam grooves. When the valve members are in a neutral position both pressure dam grooves are blocked by the axially movable valve member. Thus, pressure dams are formed when the valve members are in a neutral position. During axial movement of the axially movable valve member to an operating position one of the pressure dam grooves remains blocked and forms a pressure dam between the cylinder groove at high pressure and the return groove. The other pressure dam groove forms part of a flow passage which directs fluid between the cylinder groove at return pressure and the return groove. A valve mechanism is incorporated to restrict fluid communication between the source and the pressure dam groove in the latter condition.
Another type of prior art controller which forms pressure dams is shown in U.S. Pat. No. 3,385,057. The controller disclosed in this patent includes a pair of rotatable valve members, one of which is also axially movable relative to one other valve member to place the controller in an operating condition. A first pair of pressure dam grooves have pressure from the source continually directed thereto are in neutral. One of these pressure dam grooves is vented to return when the controller moves to an operating position. A second pair of pressure dam grooves are formed in grooves which are in continuous interfacing relationship with a fixed cylindrical wall of the controller housing. Relative movement of remote portions of the valve members to an operating position communicates pressure fluid to these grooves to form pressure dams.